1. Field of Invention
This invention relates to an ink jet printing method in which a water-based ink comprising, as essential constituents, water, a water-soluble organic solvent, and a water-soluble dye is ejected as minute ink droplets to perform printing.
2. Description of Related Art
Generally, in a known ink jet printing method, ink droplets are formed by various ink ejecting methods and a portion or all of the ink droplets are sprayed onto a printing medium such as paper to perform printing. There are several ink ejecting methods, such as, a method for electrostatically sucking ink, a method for mechanically oscillating or displacing ink by use of piezoelectric elements, and a method for heating ink to form an air bubble and ejecting ink by pressure built up by the air bubble.
Water-based inks, which use water as a main solvent, have been developed increasingly for use in such an ink jet printing method.
A water-based ink for ink jet printing is required not to clog ink nozzles at the tip of a printhead or an ink channel of an ink jet printer, to allow stable ink ejection, and to produce a bright-colored and high-density printed image.
As a coloring agent for the water-based ink used for ink jet printing, a water-soluble dye is generally used. Since the water-soluble dye dissolves completely in water, if impurities contained therein are reduced below a certain amount, the dye-containing ink is unlikely to clog the tip of the printhead or the ink channel of the ink jet printer, ensuring the production of bright-colored high-density prints. For this reason, various water-based inks that use a highly purified water-soluble dye as a coloring agent are commercially available for use with ink jet printers.
Since the environmental temperature at which ink jet printers are used is generally in a range from 5 to 40.degree. C., stable ink ejection and normal printing operation over such a temperature range are prerequisites for proper ink jet printing.
In addition, the ink jet printer that uses the water-based solvent is generally provided with a capping mechanism. Orifices of the printhead, when not in use, are covered by a cap to be hermetically sealed. This mechanism is provided for preventing clogging of ink nozzles and improper ink droplet trajectory due to increased ink viscosity and hardened ink at the tips of the nozzles. This capping mechanism functions effectively under usual service conditions.
However, when printing is performed under low temperature conditions using a conventional water-based ink for ink jet printing, the ink viscosity increases, and no or unstable ink ejection is caused. On the other hand, under high temperature conditions, the ink viscosity decreases, and ink splashing is caused. Thus, it is hard to produce high quality prints by normal ink ejection under low or high temperature conditions.
Further, under abnormal conditions, for example, where the printhead is left not covered by the nozzle cap for a long time, clogging of ink nozzles occurs due to increased ink viscosity, deposited ink, or hardened ink, and normal ink ejection is hardly restored.
If the temperature of the entire printhead is controlled to maintain the ink viscosity properly, the above-described problems can be solved. In this case, however, the printer will be complicated in structure and its price will increase.
An object of the present invention is therefore to solve the above-mentioned problems and to provide a water-based ink for ink jet printing and an ink jet printing method using the water-based ink, in which the ink allows normal ink ejection over an extended temperature range to produce high quality prints. Another object of the present invention is to provide a water-based ink for ink jet printing and an ink jet printing method using the water-based ink, in which the ink can readily be restored to the state ready for normal ejection even under an abnormal condition where the ink nozzles are left not covered by the cap for a long time.